Appendix C1
Appendix C Biosecurity risk assessment - Siam Weed (Chromolaena odorata)
Status
Chromolaena odorata (Siam Weed) is recognised as one of the world’s worst tropical weeds
(NT Department of Land Resource Management 2013). Siam weed is a member of the
Asteraceae (daisy family) and is native to the warmer parts of North America, Central
America and South America. It is recognised as one of the world’s 100 worst invasive
species and is a serious weed in South Africa, India, China, Indonesia, East Timor and the
Philippines (Figure 1, Figure 2) where it forms dense stands which prevent establishment of
other species, both due to competition and allelopathic effects (Lowe et al 2000).
Status: = Present, no further details, = Widespread, = Localised, = Confined and
subject to quarantine, = Occasional or few reports, = Evidence of pathogen, = Last
reported, = Presence unconfirmed, = See regional map for distribution within the country
Figure 1 Siam weed global distribution (CABI 2013)
Siam weed is one of 28 weed species on Australia’s National Environmental Alert List
because it threatens biodiversity, causes other environmental damage and has the potential
to seriously degrade Australia’s ecosystems (Department of the Environment 2013, NT
Department of Land Resource Management 2013).
Siam weed was first discovered in Australia in 1994 at Bingil Bay in Far North Queensland
(Biosecurity Queensland 2012) and is believed to have entered Australia as pasture seed
contaminant used on a grazing property in the 1960s and 1970s (NT Department of Land
Resource Management 2013). In mainland Australia, Siam weed distribution is confined to
north Queensland (Figure 2). There a nationally cost-shared eradication program, led by the
Queensland Government, has been in place since its discovery in 1994 however recent
assessment indicates that eradication is no longer technically feasible (Biosecurity
Queensland 2012). All known outbreaks are under active management with the aim of
eradicating the weed from the Australian mainland (NT Department of Land Resource
Management 2013).
Status: = Present, no further details, = Widespread, = Localised, = Confined and
subject to quarantine, = Occasional or few reports, = Evidence of pathogen, = Last
reported, = Presence unconfirmed
Figure 2 Siam weed Asia-Pacific distribution (CABI 2013)
On the neighbouring Christmas Island, Siam weed is a prohibited import. The weed was
detected in 2010 at a single site on the island’s north-east1 and was promptly treated
(Director of National Parks 2010). In mid 2013, the site was inspected and two individuals
were detected. Ongoing monitoring and control is being undertaken to ensure total
eradication of Siam Weed from Christmas Island. It may also be possible that there are other
as yet undetected infestations are present on Christmas Island.
Siam weed is well-established on the Cocos (Keeling) Islands (CKIs) and its presence there
poses a considerable risk for its establishment on North Keeling Island (Pulu Keeling
National Park). It is unclear how the species was introduced to the CKIs but the most likely
vector is human transport. The species is classified as a plant that is a quarantinable pest in
the Quarantine (Cocos Islands) Proclamation 2004. It currently grows on Home Island, West
Island and Horsburgh Island. The Western Australia (WA) Department of Agriculture and
Food are undertaking a control program on Home and West Islands (Reeves and Rayner
2010). However, there is no intention to extend the control program to Horsburgh Island at
present (B. Rayner 2013, pers. comm., 25 Oct). This risk forms part of the local pathway and
is the subject of this assessment.
1
Outbreak located on the North-South Baseline by the road side near the Grants Well turn off.
The current distribution of Siam weed and the extent of the ongoing control program on CKIs
can be seen in Figure 3.
Figure 3 Siam weed distribution Cocos (Keeling) Islands southern atoll (Reeves and
Rayner 2010)
Biology
The following knowledge of the biology of Siam Weed is available from its role as an
internationally significant weed and through control efforts in its naturalised Australian
distribution. Siam weed:

is an upright shrub growing up to 5 metres(m) tall that forms dense spreading thickets
and sprawling or scrambling plants growing up to 20 m high if supported by native
vegetation

is a plant with many slender stems that become woody at the base, its side branches are
often in pairs

is identified by its leaves that are arrowhead shaped, 50-120 millimetres (mm) long and
30-70 mm wide with characteristic veins in a “pitchfork” pattern

as the name “odorata” suggests is pungent when leaves are crushed

produces numerous seeds which can live in the seed bank for 6 years and effectively
germinate

seed dispersal occurs naturally (non-biotic) and biotically (vector transmission,
accidental and intentional introduction) both short and long distances

seeds germinate successfully in high humidity when temperatures are above 20°C
during the wet season but can also germinate with rain after dry periods (NB: has been
seen to germinate sporadically on CKIs

is perennial, with long-lived root stock consisting of a primary root and a very dense
branching network that lives for up to 10 years

after a year’s growth horizontal roots develop into a lignified storage organ which
reaches 20 centimetres (cm) diameter and re-sprouts if vegetative structures are
damaged

follows the C3 photosynthesis pathway and has a very efficient resource allocation
strategy that allows the plants to grow very rapidly after germination (up to 20 mm/day)

flowering generally occurs during the dry season and are visited by non-specialist
insects, although reproduction is often apomictic

seventy-five days after flowering fruits are ready for dispersal (up to 1 billion/hectare)

seedlings success is between 20–46% and appear in densities of >2000 m2 (CABI 2013,
NT Department of Land Resource Management 2013).
Ecology
The following aspects of its ecology are particularly relevant in terms of assessing risk of
introduction to Pulu Keeling National Park (PKNP) and its potential impact on island
biodiversity. Siam weed:

is well adapted to a wet-dry tropical climate as above ground foliage can die off during
the dry season and grow back vigorously from root stock during the wet season

grows on most soil types from sand to heavy clay, preferring rich well-drained soils

is adapted to grow in full sun and high productivity is dependent on seedlings having
maximum light

grows best in the tropics and subtropics in areas that receive >1,200 mm of rainfall
(1,000 mm with short dry season)

is limited to altitudes below 2,000 m

is opportunistic and forms pure stands when established, often in disturbed areas, such
as forested edges, grasslands, plantations, road verges and clearings

is highly competitive and once reproducing its fast growth outcompetes many species
because of its massive habit, domination of resources, mechanical pressure and
potential allopathic effect (CABI 2013, NT Department of Land Resource Management
2013).
NB: There are natural enemies (biocontrols) that have been translocated to Africa, Ghana,
Guam, Malaysia, Mariana Islands, Micronesia, South Africa and Thailand (CABI 2013). If
biocontrol was to be considered as a method of control, a risk assessment should be
completed that determines the most appropriate species relating to climatic area.
Establishment risk
There is a range of factors relevant to assessment of the risk of establishment of Siam Weed
on PKNP that are considered below.
Departure points
The following table outlines the major pathways that Siam weed may use to invade PKNP (
Table 1). It shows areas where the species is currently established those where there is the
potential for naturalised distribution, the potential pathways and the risk of introduction. Siam
Weed’s range does not currently extend to PKNP; however it is foreseeable the island could
form part of the future range of this species as the conditions are high suitable as outlined
below. The following information has been used to identify where threat prevention work is
needed.
As the species is already established in the CKIs the risk of introduction by sea conveyance
via the local pathways is high for islands where the species is already naturalised. If effective
threat prevention strategies including quarantine protocols were introduced and carried out,
the risk of introduction could be lowered.
Currently, there are no direct domestic conveyance pathways, either air or sea, between the
mainland and PKNP nor is the species naturalised at any of the domestic origins points. For
these reasons the risk via domestic pathways has been identified as low.
Table 1 Siam weed invasive pathways and risk
Origin
Perth (Australian Mainland)
Broome (Australian Mainland)
Darwin (Australian Mainland)
Fremantle (Australian Mainland)
Christmas Island2
Cocos Home Island
Cocos Horsburgh Island
Cocos West Island
Remaining islands within the
southern atoll
Indonesia3
Sri Lanka3
Est.?
N
N1
N1
N
N1
Y
Y
Y
Pathway
Air
Sea
Y
N
Y
N
Y
N
N
Y
Y
Y
N
Y
N
Y
N
Y
Likelihood of
introduction
Very unlikely
Very unlikely
Very unlikely
Very unlikely
Very unlikely
Possible
Possible
Possible
N1
N
Y
Unlikely
Y
native
N
N
Y
Y
Unlikely
Unlikely
1
Potential for naturalised distribution (Biosecurity Queensland 2011).
Detected on Christmas Island in 2010 and control commenced immediately after. The only known
infestation is considered to be under control (Director of National Parks 2010).
3 Suspected Illegal Entry Vessels (SIEVs)
2
The risk of introduction by SIEVs via the international sea pathways is less certain due to the
lack of ability to control its arrival using threat prevention strategies, thus the risk has been
identified as medium.
Transportability
Siam weed seed is highly transportable by humans and animals via almost any vector
including vehicles, clothing, tents and other equipment. Unintended transport of vegetative
material is less likely to occur but there is a possibility that seeds could be transported in
uncleaned equipment emanating from areas naturalised areas, such as Horsburgh, West
and Home islands. The frequency of trips by Parks staff to Horsburgh Island and the
transport of clothing and equipment between the islands is cause for concern and with
inadequate cleaning of equipment, the transport of seeds between the two islands is likely.
Climatic suitability
Siam weed is well established in the CKIs which now forms part of its naturalised range. The
tropical, high-humidity climate and distinct wet season of PKNP closely match that of Siam
weed in its naturalised global range. Hence, the climate of PKNP is considered highly
suitable.
Habitat suitability
Siam weed has the potential to become a harmful invasive species in natural forests in the
tropics (CABI 2013). Although the species prefers disturbed areas, it is predicted that the
weed will still thrive on forested edges slowly encroaching into clearings, the launch
campsite, at igloos, in survey transect areas and where there are breaks in the canopy with
readily available light. The island’s extensive survey network would likely facilitate its spread
island-wide if the species were to establish. The potential behaviour of Siam weed on PKNP
is currently unknown but the islands habitat suitability and the lack of limiting factors are a
source of concern. The known habitat preferences and presence of Siam weed in the CKIs,
combined with highly fertile soils on PKNP suggest that habitat is highly suitable.
Impact on biodiversity
The principal biodiversity impacts of Siam weed in its naturalised range in Australia stem
from its establishment of monocultures which out-compete native vegetation. The current
impact on Horsburgh Island is unquantified and there is no control program in place and the
impacts to biodiversity cannot be calculated. On Horsburgh the soil type and vegetation
structure and types are significantly different to those on PKNP as the island does not have
extensive forested areas. One could deduce that Horsburgh would succumb too many of the
same impacts as PKNP (listed below) if the distribution of Siam weed became widespread.
Pulu Keeling National Park
There is a high likelihood of the following adverse biodiversity impacts occurring if Siam
weed became invasive at PKNP:



It could significantly change the structure of the Pisonia forest community by outcompeting the native vegetation. Cocos nucifera (coconut) is one of the main host
plants affected in plantations and is abundant on PKNP (CABI 2013). Its impacts to
other species are unconfirmed but could be major or catastrophic
It would indirectly impact the EPBC listed migratory birds and the endemic
subspecies Gallirallus philippensis andrewsi (Cocos Buff-banded rail) that nest on
PKNP. The extent of future impacts/consequences are uncertain but could be
major2
The lagoon on PKNP is listed as a Ramsar wetland of international importance and
the establishment of Siam weed would be expected to have an adverse impact on
the ecological character of the wetlands via disruption to ecological processes. If the
species were to establish at the lagoon site the impacts would be catastrophic.
In the event that Siam weed established on PKNP, the benefits provided by its complete
eradication would be greater than the off-target impacts of the control program. Any control
program would use methods that do not compromise and minimise impacts on the islands
natural values including its biodiversity.
The natural values and threatened flora and fauna of PKNP would be placed at threat if the
species were introduced as this species would readily establish in and dominate a variety
of habitat types. Based on this, you could expect a range of adverse biodiversity impacts if
the species naturalised and that consequences could be majorcatastrophic.
Conclusions
Siam weed is highly transportable as seed and Parks, local and international vessel arrivals
could carry plant material from areas where it is established. The high frequency of visits by
Parks Australia staff between Horsburgh Island and PKNP greatly increases the chances of
2
Horsburgh Island is the location of the translocated population of CBBRs. The interactions between
Siam weed and the CBBR is not clear, however, could be detrimental to the survival of the population.
Siam weed control is not currently planned for Horsburgh Island.
Siam weed introduction to the park. The introduction of Siam weed to PKNP via the local
pathway has been assessed as likely. Therefore, adequate threat prevention measures are
needed to prevent incursions.
The prolific biological success of the species in many habitat types is of concern.
Additionally, its ecological requirements including climatic conditions correlate with the
environment on PKNP. Additionally, the species has established with ease on Horsburgh,
Home and West islands in the southern atoll. If introduced the establishment of Siam weed
on PKNP is assessed as almost certain.
The consequences of Siam weed introduction and establishment on PKNP have been
assessed as major and catastrophic respectively. This is based on the availability of large
areas of suitable habitat including forest edges and clearings and the potential impacts to the
values of PKNP.
Based on the above assessments, introduction of Siam weed via the local pathway
represents a VERY HIGHEXTREME biosecurity risk to PKNP.
The outcomes of this assessment are summarised in matrix below.
SIAM WEED INTRODUCTION AND ESTABLISHMENT RISK MATRIX
Consequences
Likelihood of Introduction
Almost certain
Will probably occur once in 1
month
Insignificant
Minor
Moderate
Major
Catastrophic
Low
Medium
High
Very
High
Extreme
Low
Medium
High
Very
High
Very High
Low
Low
Medium
High
High
Minimal
Minimal
Low
Medium
High
Minimal
Minimal
Low
Low
Medium
Likely
Will probably occur once in 6
months
Possible
Will probably occur once in 2
years
Unlikely
Will probably occur once in 10
years
Very Unlikely
May occur only once in 50
years or more
Likelihood of
Establishment
Almost certain
Will probably occur once in 1
month
Consequences
Insignificant
Minor
Moderate
Major
Catastrophic
Low
Medium
High
Very
High
Extreme
Low
Medium
High
Very
High
Very High
Low
Low
Medium
High
High
Minimal
Minimal
Low
Medium
High
Minimal
Minimal
Low
Low
Medium
Likely
Will probably occur once in 6
months
Possible
Will probably occur once in 2
years
Unlikely
Will probably occur once in 10
years
Very Unlikely
May occur only once in 50
years or more
References
Biosecurity Queensland (2011). Siam weed Chromolaena odorata. Brisbane, Biosecurity
Queensland: 49.
Biosecurity Queensland. (2012). "Siam weed ", from
http://www.daff.qld.gov.au/4790_7353.htm.
CABI. (2013). "Chromolaena odorata (Siam weed)." Invasive species compendium
Retrieved 1 November 2013, from
http://www.cabi.org/isc/?compid=5&dsid=23248&loadmodule=datasheet&page=481&site=14
4.
Department of the Environment. (2013). "Invasive weeds alert list." Retrieved 1 November
2013, from http://www.environment.gov.au/biodiversity/invasive/weeds/weeds/lists/alert.html.
Director of National Parks (2010). Christmas Island Weed Management Plan 2010-2015.
Internal Plan. Christmas Island, Director of National Parks.
NT Department of Land Resource Management. (2013). "Siam weed (Chromolaena
odorata)." Retrieved 1 November 2013, from http://www.lrm.nt.gov.au/natural-resourcemanagement/weeds/find/?a=13154.
Reeves, A. and B. Rayner (2010). Siam Weed (Chromolaena odorata) Control on Cocos
(Keeling) Islands, Department of Agriculture and Food (WA).